1. Field of the Invention
The present invention relates to a tin-doped indium oxide thin film and a method for making the tin-doped indium oxide thin film.
2. Description of the Prior Art
Although theoretical basis for current available solar cells is well established, research and development on new materials, structures and manufacturing processes is necessary to significantly improve solar cell efficiency.
Sunlight after penetrating earth atmosphere roughly comprises 5% ultraviolet and 45% visible and infrared light. Most absorbing materials are too narrow to effectively absorb light across such a broad spectrum. Therefore, one method for improving solar cell efficiency is converting sunlight into narrower spectrum light that is absorbable for a semiconductive absorbing material. Two known solutions are commonly implemented.
A first known solution is to provide a laminated solar cell that has more than two accumulating absorbing layers. An upper layer absorbs a short wavelength sunlight and a lower layer absorbs long wavelength portions to cover the spectrum of and to fully utilize sunlight.
The first known solution may be a laminated solar cell made from III-V group materials. However, due to extremely high cost, laminated solar cells made from III-V group materials are generally limited to extraterrestrial applications rather than generally practical industrial applications.
A second known solution is to use luminescent solar concentrators (LSC). An LSC is made from a luminescence converting material that absorbs ultraviolet or infrared wavelengths and emits visible light either up or down-converted from solar infrared or ultraviolet light, respectively. The semiconductive absorbing material then absorbs the visible light emitted by the LSC.
An LSC solar cell is less efficient than a laminated solar cell due to inefficiencies during light-conversion. Nevertheless, an LSC solar cell that generally employs only one layer of solar cell usually costs less than a laminated solar cell that has multiple layers of solar cells. The second known solution is considered a low-cost solution for raising efficiency of solar cells.
As mentioned above, an LSC is used to achieve better absorption of ultraviolet and infrared light. According to known literatures, in most cases, LSC's are externally mounted onto absorbing materials of solar cells. For example, an up-converting LSC thin film and a down-converting LSC thin film may be respectively attached to the top and the bottom of an LSC solar cell to allow the absorbing material of the solar cell to utilize both the ultraviolet portion and the infrared portion of sunlight.
However, visible light emitted by the LSC thin films first passes through multiple interfaces before reaching the absorbing material. Scattering, reflection and total reflection at each interface decreases intensity of the visible light that reaches the absorbing material. Besides, additional processes necessary for making the LSC thin films increases manufacture time and cost.
Literatures discussing application of rare earth elements on semiconductors focus primarily on thin-film electroluminescent (TFEL) devices. Conventional electroluminescent devices can be categorized into two groups, one of which is related to luminescent powders and luminescent thin films and the other is characterized by being driven with alternating current (AC).
AC driven thin-film electroluminescent devices (ACTFEL) and AC driven powder electroluminescent devices are commercially available in relevant industries. An ACTFEL device has a metal-insulator-semiconductor-insulator-metal structure formed on a basal plate. In other words, the ACTFEL device has an insulator-semiconductor-insulator (DSD) structure mounted between two electrodes.
Conventional rare earth element-doped transparent conductive thin films are, as reported in literatures, doped with single rare earth element. For example, indium oxide (In2O3) thin films doped with europium ion (Eu3+) or tin oxide thin films doped with europium ion. It is known that a thin film having tin-doped indium oxide (ITO) as a basic material and doped with europium ion does not emit strong luminescence. Furthermore, since indium oxide and tin oxide is less conductive than ITO, the luminescent indium oxide thin films doped with europium ion and tin oxide thin films doped with europium ion do not exhibit high conductivity.
Dimple P. Dutta, as published in Journal of Physical Chemistry C, 112 page 6781-6785, indicated that The photoluminescence (PL) spectra of In2O3 nanoparticles showed peaks in visible region characteristic of shallow traps present within the nanoparticles. Weak luminescence was observed in europium doped indium oxide nanoparticles.
Experimental results of PL, The absence of In2O3 host band in the excitation spectrum of Eu3+ indicates that there is almost no energy transfer from the In2O3 host to the doped Eu3+, the results show that In2O3 on the Eu ions, is not a good light-emitting host material.
Do Hyung Parki, as published in Journal of the Electrochemical Society, 153 page H63-67, indicated that a luminescent conductive thin film can be obtained by direct radio frequency sputtering a conductive tin oxide material and luminescent europium oxide material onto a basal quartz plate. Do Hyung Parki also mentioned that raising concentration of europium ion leads to concentration quenching and reduction of resistivity.
Experimental results of PL-measuring a powder structure and a thin film structure made with an optimal factor of Eu 1 atom % divulge that the powder structure and the thin film structure indeed emit light. Excited wavelength of the powder structure is 30 nm longer than that of the thin film structure, which is due to formation of different crystal morphology and tendency of the thin film structure to have more bulk properties than the powder structure.
Do Hyung Parki also disclosed a luminescent TCP transparent conductive layer. The TCP layer emits light, which is similar to luminescent powder and TCO, a transparent conductive material, when electrically stimulated. A recent TFEL device comprises a luminescent powder and a TCP layer. Both the luminescent powder and the TCP layer emit light when being excited by electrons and thus intensity of total luminescence is raised. However, the means disclosed by Do Hyung Parki failed to raise the low conductivity of a thin film doped with only europium ion.
R. Kudrawiec, as published on Materials Science and Engineering B, 105, page 53-56, 2003, disclosed a europium ion doped indium oxide thin film made on a basal plate of Silicon, quartz and porous anodic alumina (PAA) using a dry-gel method. Thin films of R. Kudrawiec on different kinds of basal plates emit light at 615 nm when exited by beams at 275 nm. However the thin film provided by R. Kudrawiec exhibits a low transparency for ultraviolet as well as a low conductivity.
To overcome the shortcomings, the present invention provides a tin-doped indium oxide thin film and a method for making same to mitigate or obviate the aforementioned problems.